The publications and other materials referred to herein to illuminate the background of the invention and, in particular cases, to provide additional detail respecting its practice are incorporated herein by reference, and, for convenience, are numerically referenced and grouped in the appended bibliography.
Recombinant DNA Technology
With the advent of recombinant DNA technology, the controlled microbial production of an enormous variety of useful polypeptides has become possible. Already in hand are bacteria modified by this technology to permit the production of such polypeptide products as somatostatin (1), the component A and B chains of human insulin (1), human proinsulin (2), thymosin alpha 1 (3), human growth hormone (4), human (5) and hybrid (6) leukocyte and fibroblast (7) interferons, as well as a number of other products. The continued application of techniques already in hand is expected in the future to permit bacterial production of a host of other useful polypeptide products, including other hormones, enzymes, immunogens useful in the preparation of vaccines, immune modulators and antibodies for diagnostic and drug-targeting applications.
The workhorse of recombinant DNA technology is the plasmid, a non-chromosomal loop of double-stranded DNA found in bacteria and other microbes, oftentimes in multiple copies per cell. Included in the information encoded in the plasmid DNA is that required to reproduce the plasmid in daughter cells (i.e., an "origin of replication") and, ordinarily one or more selection characteristics such as, in the case of bacteria, resistance to antibiotics, which permit clones of the host cell containing the plasmid of interest to be recognized and preferentially grown under selective conditions. The utility of plasmids lies in the fact that they can be specifically cleaved by one or another restriction endonuclease or "restriction enzyme", each of which recognizes a different site on the plasmid DNA. Thereafter heterologous genes or gene fragments may be inserted into the plasmid by endwise joining at the cleavage site or at reconstructed ends adjacent to the cleavage site. DNA recombination is performed outside the cell, but the resulting "recombinant" plasmid can be introduced into it by a process known as transformation and large quantities of the heterologus gene-containing recombinant plasmid are then obtained by growing the transformant. Moreover, where the gene is properly inserted with reference to portions of the plasmid which govern the transcription and translation of the encoded DNA information, the resulting expression vehicle can be used to actually produce the polypeptide sequence for which the inserted gene codes, a process referred to as expression.
Expression is initiated in a region known as the promoter which is recognized by and bound by RNA polymerase. The polymerase travels along the DNA, transcribing the information contained in the coding strand from its 5' to 3' end into messenger RNA which is in turn translated into a polypeptide having the amino acid sequence for which the DNA codes. Each amino acid is encoded by a nucleotide triplet or "codon" within what may for present purposes be referred to as the "structural gene", i.e., that part which encodes the amino acid sequence of the expressed product. After binding to the promoter, the RNA polymerase, transcribes a 5' leader region of messenger RNA, then a translation initiation or "start signal" (ordinarily ATG, which in the resulting messenger RNA becomes AUG), then the nucleotide codons within the structural gene itself. So-called stop codons are transcribed at the end of the structural gene whereafter the polymerase may form an additional sequence of messenger RNA which, because of the presence of the stop signal, will remain untranslated by the ribosomes. Ribosomes bind to the binding site provided on the messenger RNA, and themselves produce the encoded polypeptide, beginning at the translation start signal and ending at the previously mentioned stop signal. The resulting product may be obtained by lysing the host cell and recovering the product by appropriate purification from other microbial protein or, in particular instances, possibly by purification from the fermentation medium into which the product has been secreted.
Plasmids employed in genetic manipulations involved in the construction of a vehicle suitable for the expression of a useful polypeptide product are referred to as DNA transfer vectors, Thus, employing restriction enzymes and associated technology, gene fragments are ordered within the plasmid in in vitro manipulations, then amplified in vivo in the transformant microbes into which the resulting, recombinant plasmid has been `transferred`. A "DNA expression vector" comprises not only a structural gene intended for expression but also a promoter and associated controls for effecting expression from the structural gene. Both transfer and expression vectors include origins of replication. Transfer vectors must and expression vectors may also include one or more genes for phenotypic selection of transformant colonies.
Thus far, the useful products of expression from recombinant genes have fallen into two categories. In the first, a polypeptide having the amino acid sequence of a desired end product is expressed directly, as in the case of human growth hormone and the interferons referred to above. In the second, the product of expression is a fusion protein which includes not only the amino acid sequence of the desired end product but also one or more additional lengths of superfluous protein so arranged as to permit subsequent and specific cleavage away of the superfluous protein and so as to yield the desired end product. Thus, cyanogen bromide cleavage at methionine residues has yielded somatostatin, thymosin alpha 1 and the component A and B chains of human insulin from fusion proteins; enzymatic cleavage at defined residues has yielded beta endorphin (8).
A "biocompetent polypeptide", as that term is used herein, refers to a product exhibiting bioactivity akin to that of a polypeptide innately produced within a living organism for a physiological purpose, as well as to intermediates which can be processed into such polypeptides, as by cleavage away of superfluous protein, folding, combination (as in the case of the A and B chains of human insulin), etc.
Saccharomyces cerevisiae
The cells of Saccharomyces cerevisiae, or yeast, are, like those of mammalian organisms, eukaryotic in nature as distinguished from the prokaryotic nature of bacteria. With regard to mechanisms for the expression of genetic information, eukaryotes are distinguished from bacteria by:
(1) chromosomes which are organized in 140 base pair units, each containing two molecules each of histones H2A, H2B, H3, and H4. PA1 (3) Post transcriptional addition of Gppp and polyadenylic acid to the 5' and 3' termini of mRNA molecules. PA1 (4) Transport of newly completed mRNA from the nuclei where they are transcribed to the cytoplasm where they are translated. PA1 (5) Some but not all eukaryotic genes contain intervening sequences (introns) which make them non-colinear with the corresponding mature mRNA molecule. The initial transcription products of these genes contain the intron sequence which is spliced out subsequently in the formation of a finished mRNA molecule.
(2) Transcription of the protein-encoding gene by the alpha-amanitin sensitive RNA polymerase II.
The nucleotide sequences of all eukaryotic cells are transcribed, processed, and then translated in the context described above. There are reasons to believe that expression of eukaryotic genes may proceed with greater efficiency in yeast than in E. coli because yeast is a eukaryote cell.
A number of workers have previously expressed, or attempted to express, foreign genes in yeast transformants. Thus, attempted expression from a fragment comprising both a promoter and structural gene for rabbit globin is reported (9) to have yielded partial mRNA transcripts, seemingly unaccompanied either by translation into protein or maturation (intron elimination) of the message. A gene coding for Drosophila GAR tranformylas (yeast ADE8), an enzyme in the adenine synthesis pathway, is reported to have been expressed under the control of its own promoter (10). A number of yeast proteins have hitherto been expressed in yeast via recombinant plasmids (see, eg., 12). In the experiments, as in the Ade-8 case earlier discussed, expression occurred under the selective pressure of genetic complementation. Thus, each expression product was required for growth of the host strains employed, mutants whose chromosomal DNA was defective in the structural gene(s) from which expression occurred.
The availability of means for the production in yeast of proteins of choice could provide significant advantages relative to the use of bacteria for the production of polypeptides encoded by recombinant DNA. Yeast has been employed in large scale fermentations for centuries, as compared to the relatively recent advent of large scale E. coli fermentation. Presently, yeast can be grown to higher densities than bacteria, and is readily adaptable to continuous fermentation processing. Many critical functions of the organism, e.g., oxidative phosphorylation, are located within organelles, and hence not exposed to the possible deleterious effects of the organism's overproduction of foreign proteins. As a eukaryotic organism, yeast may prove capable of glycosylating expression products where important to enhanced bioactivity. Again, it is possible that as eukaryotic organisms, yeast cells will exhibit the same codon preferences as higher organisms, tending toward more efficient production of expression products from mammalian genes or from complementary DNA (cDNA) obtained by reverse transcription from, e.g., mammalian messenger RNA. Until the present invention, however, attempts to produce biocompetent expression products other than those required for cellular growth have proven largely unsuccessful.